Space Fan News #80: Fermi's Cosmic Fog; Most Distant Supernova

Uploaded by tdarnell on 02.11.2012

Hello Space Fans and welcome to another edition of Space Fan News.
Sometimes it just blows me away the things we can figure out.
Astronomers using NASA's Fermi Gamma-ray Space Telescope have made a very accurate measurement
of starlight in the universe
(That's from all stars in the universe. Ever)
We can do that?
The way they did this is actually pretty clever.
As you know, the universe still contains within it, all of the light that has ever shone from
every star throughout the history of universe.
With the exception of course, of those photons that have been sucked into black holes, but
that's a tiny percentage compared to all of the photons ever emitted.
You can think of all this light as a cosmic fog that permeates the universe. It's very
dim, but it is there and if we can measure this fog, it's density and homogeneity, things
like that, then we can get an idea of how many stars there were shining throughout all
of cosmic history.
I touch on this topic a little in a video I made a while back called, "Why is the Sky
Dark at Night" so check it out to learn more about this.
So the problem is, how do we measure this fog? If we could pass a beam through this
fog, we could measure it, kind of like seeing the light falloff from a lighthouse in the
middle of a fog bank.
This is where the clever part comes in: what beams can we use? What cosmic lighthouses
are there that we can pass through this fog to measure it.
The answer, use the most distant and brightest beams in all of creation: gamma ray bursts.
It happens like this: the optical and ultraviolet light from stars continues to travel throughout
the universe even after the stars die, and this creates a fossil radiation field we can
explore using gamma rays.
The gamma rays come primarily from blazars, distant galaxies containing active, feeding
supermassive black holes.
Gamma rays are the most energetic form of light and since Fermi's launch in 2008, its
Large Area Telescope (LAT) observes the entire sky in high-energy gamma rays every three
hours, creating the most detailed map of the universe ever known at these energies.
And these energies are huge, we're talking energies greater than 3 billion electron volts,
or more than a billion times the energy of visible light.
The total sum of all starlight in the cosmos is known to astronomers as the extragalactic
background light (EBL), and as I said before, to gamma rays, this EBL acts as a kind of
cosmic fog.
By looking at the way gamma ray light is dispersed, or attenuated, in a cloud like this, you can
learn a lot about the fog's characteristics.
The process goes like this:
As matter falls toward a galaxy's supermassive black hole, some of it is accelerated outward
at almost the speed of light in jets pointed in opposite directions. When one of the jets
happens to be aimed in the direction of Earth, the galaxy appears especially bright, that's
a blazar.
Gamma rays produced in blazar jets travel across billions of light-years to Earth. During
their journey from various periods in the history of the universe, the gamma rays from
these blazars will pass through all of the light emitted by the stars in it's path along
the way.
The more stars there are, the more light they have to pass through, which acts as an increasing
fog of visible and ultraviolet light emitted by stars that formed throughout the history
of the universe.
And the thickness of this fog (which is just starlight) is directly related to the number
of stars in the universe. Also embedded in this fog is information about when there were
more stars and when there might have been less.
So how are the gamma rays attenuated?
Occasionally, on it's long journey towards us, a gamma ray collides with photons from
this starlight fog and transforms it into a pair of particles -- an electron and a positron,
which is an antimatter electron.
Once this occurs, the gamma ray light is lost. In effect, the process dims the gamma ray
signal from the blazar just like fog dims a distant lighthouse.
From studies of nearby blazars, scientists determined how many gamma rays should be emitted
at different energies. More distant blazars show fewer gamma rays at higher energies -- especially
above 25 GeV -- because they've been absorbed by the cosmic fog.
The farthest blazars are missing most of their higher-energy gamma rays by the time we see
By measuring the average gamma-ray dampening across three distance ranges between 9.6 billion
years ago and today the scientists were able to get an estimate of the fog's thickness
throughout the journey of the gamma ray.
Once they had that information, then they were able to learn a lot.
First, to account for the observations seen with Fermi, they found the average stellar
density in the cosmos is about 1.4 stars every 100 billion cubic light-years, which means
the average distance between stars in the universe is about 4,150 light-years.
Second, star formation reached a peak when the universe was about 3 billion years old
and has been declining ever since.
So there you go, clever, huh? One could even say…..
Just Like Downtown.
The Fermi EBL measurements allow astronomers to constrain the number of stars that have
ever shone in the universe. Moving forward, the measurements will serve as a guide for
theoretical studies of the early universe, near the peak of star formation, 3 billion
years after the Big bang.
Next, astronomers using the Keck Telescope have found the most distant supernovae to
Now if you've watched 'First Light', a video we produced over the summer which showed,
among other things, what the first stars in the universe were like and how they died,
then you know about something called a pair-instability supernova.
These are extremely bright, roughly 10-100 times brighter than any other supernova type.
They occur when very massive stars roughly 100 to 250 times more massive than the Sun,
explode. Astronomers call them super luminous supernovae.
Super-luminous supernovae were discovered only a few years ago, and are rare in the
nearby Universe.
Their origins are not well understood, but a small subset of them is thought to occur
when extremely massive stars undergo a nuclear explosion triggered by the conversion of photons
into electron–positron pairs. Such events are expected to have occurred more frequently
in the early Universe (at high redshift), when massive stars were more common.
To learn more about them, watch the 'First Light' video.
These supernovae are important, because they are the deaths of the first generations of
stars that began to generate the heavier elements found in the universe today and are required
if life is to eventually evolve.
According to the press release, the team searched through a large volume of the Universe at
z greater than or equal to 2, and found two super-luminous supernovae, at redshifts of
2.05 and 3.90 — breaking the previous supernova redshift record of 2.36, and implying a production
rate of super-luminous supernovae at these redshifts at least 10 times higher than in
the nearby, and recent, Universe.
Although the spectra of these two objects make it unlikely that their progenitors were
among the first generation of stars, the results suggest that detection of those stars may
not be far from our grasp.
Of course, JWST will help with that, you can bet on it.
Well, that's it for this week Space Fans, thanks for watching and as always, Keep Looking